1. Field of the Art
This invention relates to a highly absorbent resin which is essentially a polyamine-crosslinked, partial neutralization product obtained by reaction of an alkali metal hydroxide with a carboxyl-containing polymer selected from the group consisting of alpha-olefin-maleic anhydride copolymers, vinyl compound-maleic anhydride copolymers, polyacrylic acid, polymethacrylic acid, and mixtures thereof, said resin being capable of absorbing at least 20 times its own weight of distilled water and having good durability and good heat resistance in the hydrous condition.
2. Description of the Prior Art
Apart from those materials capable of absorbing at most 10 and odd times their own weight of water, including synthetic resins useful as ion exchangers or ion adsorbers (see, for example, Japanese Patent Application Kokai No. 50-78,583), fibrous materials (e.g. absorbent cotton, wool), pulp, and porous particulate or granular inorganic materials (e.g. porous active carbon, vermiculite), hydrophilic polymeric materials capable of absorbing at least 20 times their own weight of distilled water have recently become subjects of increasing interest because of their high water-absorbing capacity. In particular, those polymer materials that can, on contact with water, absorb a very large amount of water rapidly in a short time are expected to be useful in making sanitary napkins or tampons, diapers, disposable towels for kitchen use, and soil conditioners, for instance. As such polymer materials, there have recently been proposed several materials derived from natural or synthetic high molecular substances, such as crosslinked polyoxyethylene oxide, crosslinked polyvinyl alcohol, hydrolyzate of starch- or cellulose-polyacrylonitrile graft copolymer and crosslinked carboxymethylcellulose. These highly absorbent resins are now used not only in disposable hygienic and household articles, such as sanitary napkins, tampons, diapers and kitchen articles, but also in the industrial field, for example as water leak stoppers and dehydrating agents, which make good use of water swellability of the resins, and in the field of civil engineering and construction industry, for example as soil conditioners and dew formation inhibitors. Accordingly, the requirements to be met by the highly absorbent resin are that the physical properties of the resins in the hydrous condition, that is the physical properties of the hydrous gels, do not alter upon standing for a long period of time in the hydrous condition, that the resins can endure hot water for a long period, that the resins have good heat resistance, and that the resins are resistant to acids and alkalis. However, it is no exaggeration to say that none of the above-mentioned commercially available resins and the resins under development has all of these physical properties including durability. Thus, for instance, the highly absorbent resins derived from starch or carboxymethylcellulose, when left standing in the hydrous condition, easily undergo biodegradation or decay and as a result a sufficient degree of durability cannot be expected in them. The highly absorbent resins derived from synthetic high molecular substances, such as those consisting of copolymers of a hydrophilic vinyl monomer (e.g. acrylic acid, methacrylic acid) and diacrylate or dimethacrylate of a polyol (e.g. ethylene glycol) (Japanese Patent Application Kokai No. 55-99,986) and those consisting of urea resin- or melamine resin-crosslinked alpha-olefin-maleic anhydride copolymers (Japanese Patent Application Kokai No. 54-94,525), mostly contain ester or amide bond-containing crosslinks, and consequently are susceptible to hydrolysis in the hydrous condition or in the hot hydrous condition. The hydrolysis leads to breakage of the crosslinks, and, in most cases, the initial properties of hydrous gels cannot be retained for a long period. The highly absorbent resins disclosed by Japanese Patent Application Kokai No. 53-25,666 or U.S. Pat. No. 4,155,957, which are prepared by dissolving a lower olefin-maleic anhydride copolymer in ammonia water, adding thereto a compound containing at least two hydroxyl groups (e.g. ethylene glycol, propylene glycol, glycerol), a compound containing at least two amino groups (e.g. ethylenediamine, propylenediamine), a compound containing at least two epoxy groups (e.g. diglycidyl ether) or the like (e.g. glycidyl alcohol), together with an aqueous ethylene-vinyl acetate copolymer emulsion, and allowing the crosslinking reaction to proceed, lose their water-absorbing capacity almost completely on exposure to elevated temperatures.